Insulation diagnosis for gas insulated switching devices (GIS) and transformers has been conducted through measurement of electromagnetic waves radiated due to partial discharge. Since this electromagnetic wave measurement is mainly performed in the UHF band, it is generally called a UHF method. In general, in a gas insulated device such as a gas insulated switching device or a gas insulated power transmission line (GIL), the space between a center high-voltage conductor and a metal grounding tank is exposed to a strong electric field. Therefore, if partial discharge occurs in the interior of the device, eventually, the insulation between the center conductor and the metal tank may be broken, which may result in dielectric breakdown. Therefore, there has been employed a diagnosis method in which, at the stage of partial discharge, which is a sign of dielectric breakdown, an electromagnetic wave signal propagating through the interior of a gas insulated device is detected, and determination is made as to whether the detected signal is a partial discharge signal, to thereby determine an insulation anomaly in advance. Although various methods have been proposed for detection of such a partial discharge signal, a UHF (Ultra High Frequency) method is considered to be useful for enhancing the reliability of insulation diagnosis. In the UHF method, high-frequency electromagnetic waves mainly in the UHF band (300 MHz to 3 GHz) are detected by use of an antenna having sensitivity in this band. Since the UHF method allows for measurement of partial discharge signal with high sensitivity, application of the UHF method is presently studied for a wide range of applications and for use as a measurement method for asset management of devices.
Meanwhile, presently, for measurement of partial discharge in a shipment test for gas insulated switching devices (GIS), there is a standardized method for calibrating charge quantity on the basis of a low-frequency component in a k-Hz band (IEC60270, JECO401-1990; partial discharge measurements). IEC60270 prescribes a conventionally performed standard partial discharge measurement method in which a partial discharge pulse is detected in synchronism with the phase of AC high voltage. A plant test for gas insulated switching devices is performed on the basis of this test method. Further, consideration is given to application of the UHF method to insulation diagnosis for devices which are being operated at a local substation or the like, and moves have been made toward standardizing a charge quantity calibration method based on the UHF method. Calibration of discharge charge quantity is the most important item for insulation diagnosis, and in CIGRE TF15/33.03.05, it is stated that for early anomaly detection of a GIS monitor system, it is necessary to demonstrate a detection sensitivity corresponding to a discharge charge quantity of 5 pC throughout a GIS (CIGRE TF15/33.03.05: “PD detection system for GIS: sensitivity verification for the UHF method and the acoustic method”, Electra, No. 18, pp. 75-87, 1999). As described above, establishment of a discharge charge quantity calibration method based on the UHF method has been demanded for insulation diagnosis at a local substation and a partial discharge test for shipment at a plant.
However, heretofore, the relation between discharge charge quantity and the waveform of an electromagnetic wave measured by the UHF method has not been known. Although the correlation with the energy, amplitude, etc. of the waveform has been studied, linearity has not been observed, and no theoretical evidence exists.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2003-43094    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2003-232828    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2002-5985